The use of thermoplastic stretch wrap films for the overwrap packaging of goods, and in particular, the unitizing of palleted loads is a commercially significant application of polymer film, including generically, polyethylene. Overwrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In one procedure, the load to be wrapped is positioned upon a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Braking tension is applied to the film roll so that the film is continuously subjected to a stretching, or tensioning, force as it wraps around the rotating load in overlapping layers. Generally, the stretch wrap film is supplied from a vertically arranged roll positioned adjacent to the rotating pallet load. Rotational speeds of from about 5 to about 50 revolutions per minute are common. At the completion of the overwrap operation, the turntable is completely stopped and the film is cut and attached to an underlying layer of film employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overwrapped can be shrouded in the film while the vertically arranged film roll remains in a fixed position. Alternatively, the film roll, for example, in the case of relatively narrow film widths and relatively wide pallet loads, can be made to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods.
Another wrapping method finding acceptance in industry today is that of hand wrapping. In this method, the film is again arranged on a roll, however, it is hand held by the operator who walks around the goods to be wrapped, applying the film to the goods. The roll of film so used may be installed on a hand-held wrapping tool for ease of use by the operator.
Certain applications of stretch wrap films require that the film have superior cling characteristics in its applied stretched state and have superior slip characteristics when loaded beside other wrapped articles. These types of films are referred to as "cling/slip" films and are commonly used in the shipping of carpet and fabric rolls.
Some of the properties desired of a good stretch wrap film are as follows: good cling or cohesion properties of inside/outer surfaces, optional slip between outer layers, high puncture resistance, high tear resistance in the transverse direction, good machine direction tear resistance, good transparency, low haze, low stress relaxation with time, high resistance to transverse tear especially when under machine direction tension, producible in thin gauges, low specific gravity and thus high yield in area per pound, good tensile toughness, high machine direction ultimate tensile strength, high machine direction ultimate elongation, and low modulus of elasticity.
Physical properties which are particularly significant for the successful use of thermoplastic films in stretch wrap applications include their puncture resistance, their elongation characteristics, their toughness and their resistance to tearing while under tension. In general, tensile toughness is measured as an area under a stress-strain curve developed for a thermoplastic film and it may be considered as the tensile energy absorbed, expressed in units of ft. lbs./cu.in. to elongate a film to break under tensile load. In turn, this toughness characteristic is a function of the capacity of such films to elongate. The process of stretching the film decreases that capacity. Accordingly, the stretch wrap process will decrease the toughness of the film while it is in its stretched condition as an overwrap as compared to its unstretched form. Generally this loss of toughness is proportional to the amount of stretch imparted to the film as it is overwrapping a load of goods.
Currently, different grades of stretch wrap films are commonly marketed for different end uses according to overall film properties. For example, certain stretch wrap films having superior properties for load retention are characterized by requiring a higher force to stretch the film. However, such load retention films generally have poor puncture characteristics at such stretch conditions. On the other hand, certain stretch wrap films having superior puncture resistance properties have low load retention properties, thus limiting their use.
The present invention is beneficial in conjunction with applications, such as automated stretch wrappers that are designed for the high through-put segment of the market, that typically run in excess of 22 revolutions per minute. At these high speeds, many commercially available stretch films experience catastrophic failure due to the high strain rate.
A need exists for a superior stretch wrap film with high speed extensibility in the high speed market that will not have a catastrophic failure. A need exists to develop superior stretch wrap films characterized by having excellent load retention characteristics, puncture resistance characteristics, maximum stretch, inside to outer surface cling, and, if applicable, slip properties between outer layers and against other surfaces. Such films could be used in a wider variety of end applications and, thus, not unduly limit users of stretch wrap films to selectively choosing a film based on its properties prior to initiating a stretch wrap application.